All About Spines

Spine alignment: theories

On to John K's question: What are we hoping to do by aligning the shaft to the club?

Spine alignment: explanation

So far, we have some experimental data (some more anecdotal than others) that spine alignment "helps". But why does spine introduce problems, and why should alignment help?

Stated in terms of scientific inquiry, we now have a few laws about spine alignment. They are based on occasionally contradictory evidence, but at least they show definite trends. Now we need a theory to explain those laws.

Here are the theories I have heard or have come up with myself. Let's see how the theories answer our question, in light of the evidence at hand. For the theories below, I have assigned thumbs-up or -down based on how each agrees with the empirical evidence -- and occasionally a thumbs-down because it doesn't address the question we are trying to answer.

Theory 1: stable at impact

This one I would characterize as "the common wisdom". It is generally stated, "Of course you want the shaft to have its most stable position in the target line at impact. And that means the NBP in the target line; just look at what happens to the shaft in a spine finder."

I don't get it! Maybe because I don't find it rational.
Arguments for and against this theory:

Intuitive and hand-waving. No physics involved. Basically, it is a statement rather than a theory, and does not address John's original question. It doesn't say why you should want it in a "stable position", just states that you should.

If true, then spine-to-target should be a very bad position, because the spine is among the least stable positions in a spine finder. But it's not; in fact, a sizable minority feel it's the best position. So does the most successful company in the spine alignment business.

Theory 2: NBP in target plane reduces torque needed to square clubface

This is a more mechanically rigorous statement of Theory #1. It says why it might be a good idea to have the most stable direction of the shaft in the target line.

The basis of the theory is that feel-finding may not be all wet. And it isn't. Feel-finding just has the problem that directional stiffness isn't the only thing that can cause a shaft to snap to one position and stay there; residual bend affects the position, too.

The shaft in the picture is flexed in the direction of the red arrow. That is not the same direction as the NBP. We know from intuition and experience that the shaft wants to snap so that the NBP is aligned to the direction of the bend. The word "wants" implies a torque on the shaft that rotates it into a position so the NBP is aligned with the bend.

What does this have to do with spine alignment? Well, the usual advice derived from this observation is that you want to align the NBP in the 9-3 plane, where it will be down the target line at impact. The motivation is so that the torque on the shaft is in a direction to get the clubface square, perpendicular to the target line. It guides the club rotation during release, so that the clubface is square at impact.

Arguments for and against this theory:

In order for this theory to work, the shaft bend must be in the target plane (the swing plane) during the period coming into impact -- say, the last 20 milliseconds before impact, as the hands are squaring the clubface. But we know that this is not true; toe droop is at least as important as lead-lag bend during this period. In fact, the shaft bend at impact is between 25 and 65 away from the target line, depending on the golfer and the golf club. And that is the closest it gets during those last 20 milliseconds before impact.

If this theory is correct, then the flight differences due to a misaligned shaft would be the problems of an un-square clubface. But the Butler study found that the problems of a misaligned shaft are more consistent with a larger dispersion of impact between the ball and the clubface -- the golfer misses the sweet spot by more with a misaligned shaft. This is not the problem we would expect from an un-square clubface.

If this theory is true, then alignment with the spine at 9-3 (rather than the NBP) should be a very bad position. In order to square the clubface, you'd have to rotate the shaft "uphill" -- in opposition to the torque. But most experience indicates that either the NBP or spine in the target line gives pretty good results. There are camps that argue one or the other is better, but all agree that either alignment is much better than, say, a 45 alignment.

It could validate the myths about "supershafts", since an aligned club with a large spine could be more self-squaring than a shaft without a spine. (But so far the supershaft is a myth. I know of no study, careful or otherwise, that has validated the myth.)

Theory 3: Bend causes out-of-plane motion of the shaft

This is the one I feel has the biggest likelihood of actually affecting the shot.

If, during the downswing, the shaft bends in a direction other than the spine or NBP, the spring force caused by shaft bending will not be in the plane of the bend. Because the spring constant (the stiffness) of the shaft is greater at the spine than the NBP, the force will be pointing a little to the spine side of the bend.

Out-of-plane forces will accelerate the head to out-of-plane motion. The result is that the clubhead is out of position at impact. This sort of effect is accentuated by a few things:
  • More bend. The larger the bend, the greater the spring force, thus the greater the potential for out-of-plane force.
  • Bend further away from the spine and NBP. The further the bend is from the principal planes of the shaft, the more out-of-plane the the spring force is. There is a curve showing this effect in the article on FLO physics.
  • Earlier bend. The effect depends on force, which creates acceleration. But the performance difference depends on how far out of plane the clubhead is at impact. Distance from the sweet spot is affected by acceleration, but it isn't the same thing as acceleration. Acceleration needs time to create velocity, and velocity needs time to create displacement. So there is a strong dependence -- actually a square-law dependence -- on time for a force to create a displacement. That means that the earlier in the downswing the bend occurs, the more out-of-plane the eventual displacement of the head at impact.
According to this theory, the best alignment strategy is to place either the NBP or the spine in the plane where the large, early bend occurs. That can be made to work because the large bend is early in the downswing, and pretty close to the heel-toe plane of the club. And it can work with either the NBP or the spine because those are the directions of bend that generate no out-of-plane spring forces.
Arguments for and against this theory:

It is quantifiable physics, not hand-waving.
It would explain the results of the Butler study, the major study so far, that misaligned spines can cause a bigger impact dispersion area on the clubface. An out-of-position clubhead results in an off-center impact, which is the result found by the Butler study.

It would explain why there are those advocating NBP to target and those advocating spine to target. Under this theory, they both work pretty well, so advocates of either are right. (Though it isn't "to target" that matters for this theory; it's the heel-toe plane. But now we know that if the NBP is at the target then the spine is heel-toe, and vice versa.)

Some initial analytical studies (including my own and another by Werner and Greig) show that it takes a lot of spine to give the sort of dispersion the Butler study reported. Since Butler said nothing about how big the spines were, there's no way of telling whether this theory is a match to their observations.

Theory 4: NBP aligned with the club's CG

Tom Wishon has come up with another interesting theory of what spine does and consequently how to align it. His theory goes like this:
  • In the vicinity of impact, the only force creating shaft bend is centrifugal force, which acts on the center of gravity (CG) of the clubhead. Therefore, the shaft is bending in the direction of CG. That is, the plane of bend is the plane defined by the shaft and the CG of the clubhead. So the shaft bends in the direction of the yellow arrow in the diagram.
  • If that is true, then the most stable plane of the shaft (the NBP) should be in the bending plane. So the NBP should be oriented toward the CG of the clubhead.
  • If you choose a different alignment, the bending forces might generate some torque on the clubhead which would interfere with the squaring the clubhead at impact.

Arguments for and against this theory:
It is basically like theory #2, except that it recognizes that the torque is controlled by the direction of bend -- and that direction is not in the target line at impact.
Unfortunately, it adopts another erroneous assumption about the direction of bend at impact. As we know from ShaftLab, the bend at impact is not aligned with the CG. It shows more lead than CG alone would explain, and it varies considerably from golfer to golfer. In more detail:
  • For a driver, we would expect the bend to be 16-18 from heel-toe, based on CG. Actually, it is at 37-66 for the golfers measured, with a median of 44.
  • For a 5-iron, we would expect the bend to be 10-14 from heel-toe, based on CG. Actually, it is at 24-59 for the golfers measured, with a median of 41
So alignment with the CG does not align the NBP to the bend.You would need something like ShaftLab to measure each golfer for spine alignment, if a theory like #2 or #4 is valid.

Wishon has done some computer analyses that suggest this might be a problem. And there is some limited anecdotal experience (Bernie Baymiller has reported some success with irons) in support of the theory. But there have been no actual measurments nor comparisons with other approaches.

Theory 5: It's all feel at impact and after impact

One possible explanation is that the difference between well-aligned and poorly-aligned shafts is just the feel of the club as a result of impact. When the club hits the golf ball, the shaft bends backwards very sharply and suddenly. This translates into the feel of impact one gets through the hands. There are at least two possible reasons that a poorly-aligned shaft might give a bad feel at impact:
  1. The shaft suddenly bending back has an out-of-plane force on it, unless either the NBP or the spine is in the direction that the bend was just applied.
  2. The shaft suddently bending back applies a torque that wants to twist the grip away from the current position, unless a stable direction of the shaft is aligned with the bend just applied.
Arguments for and against this theory:

This is consistent with the general wisdom that either the NBP or the spine must be in the target line. While most say NBP, a substantial minority favor the spine. Either one is consistent with reason #1 above. The NBP alignment is also consistent with #2 above.

This theory does not explain the clubface impact results from the SST/Butler study, nor other studies where actual performance differences were observed. It just addresses feel.

Is this serious to dismiss the theory? Maybe not. It is well-established that golf performance stems in part from expectations and confidence. There is no doubt that a good-feeling club breeds confidence, and a club that feels lousy at impact raises negative expectations. So a bad-feeling club, if that feel problem were due to misaligned spine, could account for performance problems as well as feel problems.

Unfortunately, none of the studies did anything to distinguish feel problems from genuine performance problems. In order to test for performance issues unrelated to feel, you have to remove expectations. That means not only double-blind studies, but only one hit per "turn" with each club, before the test subject moves on to another (blind-random) club. This removes any expectation about how the club will feel when you hit it.
Other than the SST/Butler study, most studies and anecdotal reports don't distinguish clearly between feel and performance. Some, in fact, explicitly talk about feel rather than measured performance. So a theory that explains a difference in feel explains most of the reported results.

Theory 6: Just get it consistent

Ed Reeder did a critique of a draft of this article. He suggested that I had missed a theory. The theory he proposed was, "We have no clue what's really going on. The best we can do is to minimize the possible effect of spine by aligning all the clubs the same way -- so at least they're consistent."

I don't agree with it, but it does make sense to include it for completeness. So let's give it the same scrutiny we gave the other theories.

Arguments for and against this theory:

It doesn't answer John Kaufman's original question. It doesn't address, "What fault are we trying to fix with spine alignment?" Inconsistency by itself isn't a fault. If we took a set and painted some shafts black and some white, they would be inconsistent. But, if there were any difference in performance, we'd have to admit that the differences were between the ears of the golfer using them. (Incidentally, this test has been done -- well over a decade ago, when graphite shafts were beginning to have a market presence. And yes, there were differences in performance, and they were between the ears of the golfers.)

If this theory is correct, then why do some alignments (NBP at 9-3, spine at 9-3, spine at 6-12) get consistently better reviews than either NBP or spine on the diagonal. If consistency were all there were to it, then a set consistently at 4:30-10:30 would be as good as a set consistently at 9-3. And almost all the data, both studies and anecdotes, disagree with that assertion.

Similarly, if it is just consistency, then all tests of drivers would be inconclusive. There would be no way to say any orientation is better than any other orientation, because consistency doesn't mean anything for a single club. But again almost all our data says there is a substantial difference in alignment orientations from one driver to another.

It is worth noting that, in designing a test for theories #5 or #6, robots are of no help whatsoever. The other theories might be tested either with human or robot swings, but the last two are completely dependent upon human reactions to the shaft spine. Here's an important corollary to this observation. Those (e.g.- shaft manufacturers) who argue that spine means nothing because robot testing has not shown it to mean anything do not have complete evidence for their position.

Bottom line

The only theories that might be viable answers to John's question, based on all the data I have seen, are:
  • #3, out-of-plane forces cause impact farther from the sweet spot, and
  • #5, feel at impact and after impact are significantly affected by spine alignment.
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Last modified -- 11/30/2008